(1) Field of the Invention
The present invention relates to a crank driving device for a press.
(2) Description of the Prior Art
In general, press equipment is driven by a so-called crank device. Specifically, a crankshaft is rotatably supported on a machine body. The portion with larger diameter of the connecting rod is engaged with its eccentric element, and the portion with smaller diameter is connected to the slide. When the slide is movably guided along the machine upward and downward, the slide can be moved up or down by rotating the crankshaft through means commonly practiced.
If the upper die is placed face-to-face and fixed to the slide and the lower die to the bolster, which is a part of the machine, press operation can be achieved by a pair of these dies. The length of the stroke of upward and downward movements of the slide is twice as long as the eccentricity of the crank.
To obtain higher productivity, the rate of the strokes of the press should be increased. However, with this crank device the speed of the slide increase, and this leads to higher vibration and noise of the press. Also, the press operation itself is adversely affected as the so-called drawing speed is increased.
Because the average speed of the slide can be given by the product of the stroke length (S) and the number of revolutions per minute (N) of the crankshaft, which is the number of the strokes of press. Thus, the product SN is an important reference factor in designing a press. When the value of SN is constant, the relationship between the stroke length (S) and the number of revolutions (N) can be given as shown in FIG. 3.
It is evident from this figure that the stroke length (S) should be shortened for the purpose of increasing the number of revolutions (N). As the result, a structure providing variable stroke length (S) has been proposed in the past. This is illustrated in FIG. 4.
The center of the crank shaft 22 is at Om, and the eccentric element 23 is eccentric from Om by e.sub.1. Eccentric ring 8 is engaged in the eccentric element 23. To the eccentric element 23, the eccentric ring 8 is eccentric by e.sub.2. With the eccentric ring 8, the portion with larger diameter 5U of the connecting rod 5 is engaged. By rotating the eccentric ring 8 relative to the crankshaft 22, the relative eccentricity of the eccentric ring 8 to the center (Om) of the crankshaft 22 can be changed within the range of maximum value (e.sub.1 +e.sub.2) to minimum value (e.sub.1 -e.sub.2). Therefore, the maximum value for the stroke length is 2 (e.sub.1 +e.sub.2), and the minimum value is 2 (e.sub.1 -e.sub.2). Here, press machine is operated with the eccentric ring 8 fixed on the eccentric element 23 by means commonly known.
Examples of such mechanisms are described in Japanese Provisional Patent Publication No. 59-92200, Japanese Provisional Utility Model Publication No. 56-34698, West German Patent Publication No. 3112382.1-14,.
(3) Problems to be solved by the invention
Nevertheless, the conventional type multi-layer eccentric structure has the following problems:
(1) The eccentric ring 8 is engaged with the crank unit 23, and the upper end 5U (portion with larger diameter) of connecting rod 5 is engaged with it. Because press load is also applied on the eccentric ring 8, it must have considerable mechanical strength. Accordingly, the dimensions in the radial direction are too long, and this hinders a compact design of the press.
(2) The longer radial dimension of a multi-layer eccentric structure also directly leads to an increase of inertia. As the result, larger power transmission devices such as the main gear must be larger, and this decreases in turn the rotation control. Moreover, the upper end 5U of the connecting rod 5 has a larger diameter, and the peripheral speed is increased. This requires a more sturdy construction to cope with higher acceleration. On the other hand, this hinders high-speed operation of the machine.
(3) The multi-layer eccentric structure is accommodated in the so-called crown chamber of the press. Because of the recent increased requirements in the industry such as automation, diversification of the processes, etc., the crown chamber is more and more densely occupied. This imposes strong restrictions on a larger design of the press or on modifications of the device or its layout.
(4) The multi-layer eccentric structure requires an adjusting means, which adjusts the relative angle between the eccentric element 23 of the crankshaft 22 and the eccentric ring 8, and strictly maintains the relative angle once adjusted. This further aggravates the above problems (1)-(3). Making adjustment in a narrow crown chamber causes many difficulties and inconveniences, and the demands for high efficiency production of diversified products cannot be satisfied.